The present invention relates to antenna feeds and, more particularly, millimeter wave frequency feeds adapted for low f/D reflectors.
One type of antenna known as a reflector antenna uses a contoured reflective surface to generate a highly directive far field antenna pattern. A small waveguide aperture feed antenna is typically placed at the focus of the reflector in order to illuminate the same. The desired directivity properties determine the relative dimensions of the reflector. A common parameter describing the geometric properties of a reflector antenna is f/D, which is the ratio of the focal length (f) to the diameter (D). The smaller f/D, the thinner or more compact the reflector antenna assembly can be made.
However, as one decreases f/D, the beamwidth of the illuminating feed must be increased proportionally in order to properly illuminate the reflector surface. For example, it is generally accepted that the reflector surface must receive energy from the feed in such a way that the energy level at the reflector edges is only about 10 decibels (dB) lower than the energy level at the center of the reflector.
One can obtain a broader beamwidth by decreasing the aperture size of the waveguide feed. Fundamentally, the lowest frequency of propagation in such a feed increases with decreasing rectangular waveguide width or circular waveguide diameter. The cutoff frequency of the dominant propagating mode is the waveguide""s lowest frequency of operation. In summary, as the feed beamwidth is broadened and the aperture size is decreased, the cutoff frequency of the aperture will increase. Consequently, at a particular frequency, the maximum feed antenna beamwidth is limited, and along with it, the minimum obtainable f/D of the reflector antenna.
In addition, as the desired operating frequency increases into the millimeter-wave range and the aperture size decreases, it becomes difficult to physically machine the aperture and other small structures related to controlling the resulting electromagnetic waves.
The present invention is an electromagnetic energy feed formed from a section of open-ended, dielectric filled waveguide. The dielectric fill material used is a solid, processable (e.g., machinable), low-loss material that can be shaped as desired.
The dielectric material used to fill the waveguide lowers the cutoff frequency of the dominant electromagnetic mode compared to the same waveguide filled only with air. This allows one to increase the beamwidth when compared to a similar sized, but air-filled only waveguide section.
A broadening of the beamwidth of approximately 10% over an air-filled-only feed has been observed with the propagating mode cutoff frequency set low enough to maintain a good input match. These attributes were achieved for a feed designed to operate in a millimeter wave frequency band at approximately 60 GigaHertz (GHz).
One preferred material for use as the dielectric is Rexolite(copyright). Other suitable materials could be used as long as their properties are stable with temperature and easily processable, i.e., they can be machined or shaped to the desired size to fill the waveguide.
The dielectric filled section is preferably provided as a solid fill of the interior dimension of the waveguide. However, even a partial filling of the waveguide can also be used to provide increased beamwidth.
The preferred embodiment uses a circular-type filled waveguide. However, other waveguide shapes, such as rectangular, may be used as well.
A quarter-wave choke slot may be used to encircle the dielectric-filled waveguide section. The choke slot may be used to match beamwidths in the electrical (E) and magnetic (H) planes. Because the aperture diameter of the dielectric-filled feed is smaller, a ridge between the circular waveguide and the choke slot may be thickened compared to that of an air-filled feed, making the choke slot easier to fabricate for a dielectric-filled feed than for an air-filled feed.
According to other optional aspects of the present invention, a protruding dielectric portion or tip may be used for efficient power transfer at the free space side of the feed. In this arrangement, the tip diameter is chosen to provide maximum power transfer with specific dimensions depending upon the dielectric constant of the dielectric fill. The length of the tip is chosen to be about one-quarter of the wavelength of the expected frequency of operation. In effect, the tip provides a single step, quarter wave transformer to match the feed aperture to free space.
Adaptations may also be made at the waveguide end of the feed. In particular, circular waveguide is not commonly used to construct microwave system components because of its reduced dominant-mode bandwidth compared to rectangular waveguide. Therefore, in a preferred embodiment, the input side of the feed uses a quarter wavelength waveguide transition (e.g., transformer). The transformer matches the field configuration of the circular waveguide used for the feed to the rectangular waveguide used to carry the signal.
In a preferred embodiment, the transformer is an annular ring of dielectric material. In this arrangement, the cross-sectional dimension of the annular ring transformer is chosen depending upon the interior dimension of the rectangular waveguide and the dielectric constant of the feed fill material. The dielectric ring provides an inhomogeneous, quarter wave matching section, functioning much the same as the tip used at the free space end.
It should be understood that the tip at the free space end and the annular ring at the input are specific embodiments of matching sections chosen for ease of machining. They can be interchanged or take other forms in other embodiments. For example, a dielectric tip can be used on the waveguide side, and an annular ring may be used on the free space side.
In a preferred embodiment, metal bosses are placed at the free space end of the waveguide adjacent the protruding tip. The bosses protect the protruding tip, for example, during handling of the feed while manufacturing an antenna assembly. Without the bosses, the protruding tip might otherwise be prone to breakage. The bosses are dimensioned and positioned in such a way that they do not interfere with the electromagnetic radiation properties of the feed.
Finally, the feed may be used with different types of reflectors, including standard parabolic metallic reflectors, transreflectors, and the like.